The present invention relates to a watertight and thermally insulating tank, particularly for storing liquefied gases, such as liquefied natural gases with high methane content, at a temperature of about xe2x88x92160xc2x0 C., the said tank being built into a bearing structure of a ship, particularly the hull of a ship intended for transporting liquefied gases by sea.
The present invention relates more particularly to a watertight and thermally insulating tank built into a bearing structure of a ship, the said bearing structure being of polygonal cross section and comprising a number of practically flat rigid walls adjacent via their longitudinal edges, at least one of the said walls having a variable width over at least part of the length of the wall, the solid angles of intersection of the bearing structure which are formed by the said variable-width wall and the adjacent walls being orientated obliquely.
A tank comprising two successive watertightness barriers, one of them a primary barrier in contact with the product contained in the tank and the other a secondary barrier arranged between the primary barrier and the bearing structure, these two watertightness barriers alternating with two thermally insulating barriers, a primary one and a secondary one respectively, is known, particularly from French Patent Application 2 724 623.
In the aforementioned application, the secondary barriers and the primary insulating barrier essentially consist of a collection of practically parallelepipedal prefabricated panels fixed mechanically to the walls of the bearing structure, each panel being formed, firstly, of a first rigid plate carrying a layer of thermal insulation and with it constituting a secondary insulating barrier element, secondly, of a flexible sheet adhering to practically the entire surface of the layer of thermal insulation of the aforementioned insulating barrier element, the said sheet comprising at least one continuous thin metal foil forming a secondary watertightness barrier element, thirdly, of a second layer of thermal insulation adhering to the aforementioned sheet, of a second rigid plate covering the second layer of thermal insulation and with it constituting a primary insulating barrier element. The primary watertightness barrier consists of metal strakes, for example made of Invar, held mechanically so as to slide on the rigid plate of the primary insulating barrier by their turned-up longitudinal edges.
Each prefabricated panel has the overall shape of a rectangular parallelepiped, the secondary insulating barrier element and the primary insulating barrier element having, respectively, viewed in plan view, the shape of a first rectangle and of a second rectangle, the sides of which are practically parallel, the length and/or width of the second rectangle being shorter than that (those) of the first rectangle so as to form a peripheral rim.
The peripheral rims of adjacent panels and the lateral walls of the primary insulating barrier elements define joining regions which are filled with insulating tiles each consisting of a layer of thermal insulation, covered by a rigid plate, the rigid plates of the insulating tiles and the second rigid plates of the panels constituting a practically continuous wall able to support the primary watertightness barrier, the regions where the secondary insulating barrier elements meet being filled with connectors made of insulating material. To ensure the continuity of the watertightness of the secondary watertightness barrier at the join between two panels, the rims are, before the joining tiles are fitted, covered with a band of flexible sheet comprising at least a continuous thin metal foil, the said band adhering to the adjacent lateral rims.
It is known that the cooling of the tank generates, at the primary and secondary watertightness barriers, tensile stresses which add to the tensile stresses generated in these watertightness barriers by the deformation of the beam that constitutes the ship when the ship is moving in the swell. When practically flat Invar-plate strakes are used, the movements of thermal contraction are of limited amplitude, but nonetheless remain. In a known way, the metal strakes, slideably mounted on the prefabricated panels, are fixed at their ends to the bearing structure of the ship by a rigid corner structure such as those described in French Patents 2 709 725 and 2 780 942, so that tensile forces in the longitudinal direction of the strakes are transmitted to the bearing structure.
The bearing structure to which the panels are fixed is formed from the walls of the double hull of the ship. The walls of the double hull form compartments each defined by a number of practically flat longitudinal walls adjacent by their longitudinal edges and having a polygonal cross section in the shape of a polyhedron, particularly an irregular octahedron, the angles at the solid angles of intersection of two adjacent longitudinal walls of which generally measure 90xc2x0 or 135xc2x0, and two transverse partitions at the longitudinal ends of the compartment, parallel to one another and perpendicular to the longitudinal walls. The longitudinal walls and the transverse partitions of a compartment constitute the bearing structure of the tank. In general, the longitudinal walls are arranged more or less in a cone with a polygonal directrix curve, in the bow part of the said ship and also in its stern part, and as a cylinder with a polygonal directrix curve in the remainder of the ship.
To produce a tank built into a compartment of constant cross section, comprising only rectangular longitudinal walls, the prefabricated panels are arranged side by side parallel to the axis of the tank, and the strakes are arranged longitudinally on the panels. In the case of a tank intended to be built into the front of the ship, the compartment generally has at least one bottom wall and a roof wall of trapezoidal shape, the cross section of which decreases towards the front of the ship. On these walls of trapezoidal shape, the prefabricated panels are also arranged parallel to the axis of the tank and cut to suit the oblique solid angles of intersection, the strakes being held parallel to the longitudinal axis and cut obliquely at the ends to tailor them to the oblique solid angles of intersection. The end of each strake is fixed at an angle to a vertical pillar, itself fixed to the bearing structure at the oblique solid angle of intersection. To allow this fixing, the pillar is formed of two stainless steel posts welded one on each side of an Invar mounting plate to which the strake is welded, the secondary watertightness barrier also being fixed to the said mounting plate.
A fixing such as this establishes a direct thermal bridge between the primary barrier and the bearing structure, and this is prejudicial in terms of insulating performance. Furthermore, such a structure has numerous disadvantages. Producing the pillars entails heterogeneous welding which is tricky to implement. Access to the pillars is relatively difficult and makes the operation of welding the strakes to the mounting plate painstaking. The size of the pillars makes filling the corner structures at the solid angles of intersection with insulating tiles more difficult. In addition, the pillars have a tendency to twist because the strakes are fixed to them at an angle.
The object of the invention is to propose a tank built into a bearing structure with obliquely orientated solid angles of intersection which make it possible to alleviate the aforementioned drawbacks.
To achieve this, the present invention proposes a watertight and thermally insulating tank built into a bearing structure particularly of a ship, the said bearing structure being of polygonal cross section and comprising a number of practically flat rigid walls adjacent by their longitudinal edges, at least one of the said walls having a width that varies over at least part of the length of the wall, the solid angles of intersection of the bearing structure which are formed by the said variable-width wall and the adjacent walls being orientated obliquely, the said tank comprising two successive watertightness barriers, one of them a primary watertightness barrier in contact with the product contained in the tank and the other a secondary watertightness barrier arranged between the said primary watertightness barrier and the bearing structure, a primary thermally insulating barrier being arranged between these two watertightness barriers and a secondary thermally insulating barrier being arranged between the said secondary watertightness barrier and the bearing structure, the secondary insulating and watertightness barriers and the primary insulating barrier being essentially formed of a collection of juxtaposed panels fixed to the walls of the bearing structure over practically its entire interior surface, the said panels being able to support and to hold the primary watertightness barrier, the said primary watertightness barrier comprising practically flat running metal strakes, made of thin plate with a low coefficient of expansion, the longitudinal edges of which are turned up towards the inside of the tank, each running strake being assembled in a watertight manner with at least one longitudinally adjacent running strake, the adjacent turned-up edges of the said running strakes being welded to the two faces of a weld support, which is held mechanically on panels and constitutes a sliding joint, characterized in that the said primary watertightness barrier further comprises, at each variable-width wall, one or more practically flat central strake(s) made of thin plate with a low coefficient of expansion, which is (are) arranged longitudinally and each of which is fixed to underlying panels, running strakes being held, parallel to the oblique solid angles of intersection of the variable-width wall, on underlying panels and fixed in a watertight manner at the ends to the central strake(s), so that the tensile forces experienced by the running strakes in their longitudinal dimension, generated by the thermal contraction and/or the static or dynamic pressure of the product contained in the said tank, are transmitted at least in part to the bearing structure via the central strake(s).
According to one embodiment, the variable-width wall has a plane of symmetry passing through the longitudinal axis of the said wall and perpendicular to the flat surface of the said wall.
In particular, the variable-width wall has a width which varies monotonously along the entire length of the said wall.
According to one particular feature, one or more end central strakes are fixed to the bearing structure by rigid corner structures.
According to one embodiment, the aforementioned panels comprise central panels juxtaposed longitudinally along the said plane of symmetry of the variable-width wall, forming at least one row, to which the central strake(s) are fixed, so that the transverse components of the said tensile forces experienced by the running strakes in their longitudinal dimension at least partially cancel each other out, and lateral panels arranged on each side of the central panels on which running strakes are held.
According to another particular feature, the tank comprises several central strakes, the adjacent transverse edges of the said central strakes being welded to weld supports which are held mechanically on the central panels.
According to one embodiment, the said central panels are formed, firstly, of a first rigid plate carrying a layer of thermal insulation and with it constituting a secondary insulating barrier element, secondly, of a sheet adhering to practically the entire area of the layer of thermal insulation of the aforementioned secondary insulating barrier element, the said sheet comprising at least one continuous metal foil forming a secondary watertightness barrier element, thirdly, of a second layer of thermal insulation covered by a second rigid plate and by a rigid layer which are juxtaposed, the said rigid layer and the said second layer of thermal insulation which at least partially cover the aforementioned sheet and which adhere thereto constituting a primary insulating barrier element, the said central panels being arranged in such a way that the second layers of thermal insulation and the rigid layers alternate longitudinally, the central strake(s) being at least fixed to the rigid layers of the central panels.
Advantageously, the weld support associated with two adjacent central metal strakes is held mechanically on the rigid layer of a central panel and is a section piece with a bracket-shaped cross section, one of the flanges of the bracket being fixed against the lateral face of the rigid layer facing the second layer of insulation of the central panel, while the other flange is fixed by one of its faces against the top face of the solid layer and welded by its other face to the adjacent transverse edges of the two central strakes.
According to a particular feature, the ends of the running strakes partially cover the central strake(s) and have an oblique edge practically parallel to the plane of symmetry, along which they are welded to the central strake(s).
According to one embodiment, each central panel has the overall shape of a rectangular parallelepiped, the secondary insulating barrier element and the primary insulating barrier element having, respectively, viewed in plan view, the shape of a first rectangle and of a second rectangle, the sides of which are practically parallel, the length and/or width of the first rectangle being shorter than that (those) of the second rectangle so as to form a peripheral lateral rim, preferably of constant width.
According to one embodiment, the lateral panels are formed, firstly, of a first rigid plate carrying a layer of thermal insulation and with it constituting a secondary insulating barrier element, secondly, of a flexible sheet adhering to practically the entire surface of the layer of thermal insulation of the aforementioned secondary insulating barrier element, the said sheet comprising at least one continuous thin metal foil forming a secondary watertightness barrier element, thirdly, of a second layer of thermal insulation which at least partially covers the aforementioned sheet and which adheres thereto and, fourthly, of a second rigid plate covering the second layer of thermal insulation and with it constituting a primary insulating barrier element.
According to one embodiment, the tank comprises first lateral panels having the overall shape of a rectangular parallelepiped, the secondary insulating barrier element having, viewed in plan view, the shape of a first rectangle, the primary insulating barrier element having, viewed in plan view, the shape of a second rectangle, the two rectangles having their sides practically parallel, the length and width of the second rectangle being shorter respectively than the length and width of the first rectangle, a peripheral rim, preferably of constant width, thus being formed on each first lateral panel around the primary insulating barrier element of the said first lateral panels, the said first lateral panels being arranged in one or more row(s), their longitudinal axes parallel to an oblique solid angle of intersection, and second lateral panels having, in cross section, the shape of a rectangular trapezium, the secondary insulating barrier element having, viewed in plan view, the shape of a first rectangular trapezium and having a face which is oblique with respect to the longitudinal axis of the said second lateral panels, the primary insulating barrier element having, viewed in plan view, the shape of a second rectangular trapezium and having a face which is oblique with respect to the longitudinal axis of the said second lateral panels, the two rectangular trapeziums having their sides practically parallel, the length and width of the second rectangular trapezium being shorter respectively than the length and width of the first rectangular trapezium, a peripheral rim, preferably of constant width, thus being formed on each second lateral panel around the primary insulating barrier element, the said second lateral panels being arranged between the first lateral panels and the central panels, their longitudinal axes parallel to an oblique solid angle of intersection and their oblique faces parallel to the longitudinal faces of the central panels.
Advantageously, the peripheral regions that there are between the primary insulating barrier elements of two adjacent central panels, of two adjacent lateral panels or of an adjacent central panel and second lateral panel are filled, so as to ensure the continuity of the primary insulating barrier consisting of the central and lateral panels, using insulating tiles, each of which consists of a layer of thermal insulation covered by a rigid plate, each tile having the thickness of the primary insulating barrier, so that after assembly, the rigid plates of the insulating tiles form, with the second rigid plates of the lateral and central panels and the top faces of the rigid layers of the central panels, a practically continuous wall capable of supporting the primary watertightness barrier.
According to one particular feature, the central strakes are arranged in first longitudinal setbacks present on the rigid layer and the second rigid plate of each central panel, and on the rigid plates of the tiles forming the junction between two central panels, the flanges of the weld supports of each central panel being housed in transverse setbacks of the rigid layer so that the central strakes form, with the two rigid plates and the top faces of the rigid layers of the central panels, a practically continuous surface.
Advantageously, two longitudinal bands of thermal protection are arranged, under the central strakes, on each side of the plane of symmetry, in second longitudinal setbacks present on the rigid layer and the second rigid plate of each central panel, and on the rigid plate of the tiles forming the junction between two central panels, so as to thermally protect the underlying regions during the operation of welding the running strakes to the central strakes.
According to another particular feature, the longitudinal edges of the central strakes are screwed to the rigid layer, the second rigid plate of the central panels and the plate of the joining tiles by means of screws, the heads of which lie flush with the top surface of the central strakes and are covered by the ends of the running strakes, the oblique edges of the said ends being welded beyond the said screws.
Advantageously, the central strakes comprise holes obtained by punching so as to allow the fixing screws to pass and so as to accommodate the heads of the said screws in recesses, third setbacks present on the rigid layer and the second rigid plate of each central panel and the rigid plate of the tiles forming the junction between two central panels being designed to accommodate the material upset during the punching operation and corresponding to the said recesses.
According to one embodiment, the said rigid layer consists of at least one block of plates of bonded ply.
According to one particular feature, the weld support associated with the running metal strakes of the primary watertightness barrier is a section piece with a bracket-shaped cross section, one of the flanges of the bracket being welded to the turned-up edges of two adjacent metal strakes of the primary watertightness barrier, while the other flange is engaged in slots, parallel to an oblique solid angle of intersection, which are made in the thickness of the second rigid plate of first lateral panels parallel to their longitudinal axes, in the thickness of the second rigid plate of second lateral panels perpendicular to their longitudinal axes and in the thickness of the rigid plate of joining tiles filling the peripheral regions that there are between the primary insulating barrier elements of two adjacent lateral panels and between the primary insulating barrier elements of a central panel and of a second lateral panel.
Advantageously, the central panels and the first lateral panels consist of prefabricated panels, the second lateral panels consisting of prefabricated panels cut to size at the time of fitting of the secondary barriers and of the primary insulating barrier in the region of the variable-width wall.
According to one particular feature, the layers of thermal insulation of the secondary insulating barrier elements of the central panels consist of a compressible cellular plastic and may have, parallel to their large faces, a number of fibreglass fabrics forming practically parallel leaflets so that the tensile forces of the running strakes are reacted partly by the corner structures of the bearing structure to which corner structures the end central strake(s) is (are) fixed, and partly by the variable-width wall of the bearing structure to which the central panels are fixed, the distribution of these forces depending on the flexibility of the cellular plastic used.
In one embodiment, the tank is built into the front or rear part of a ship. In particular, the bearing structure comprises at least two mutually parallel trapezoidal longitudinal walls forming the bottom and the roof of the tank.